Hello and welcome to this presentation of the STM32
independent watchdog. It covers the main features of
this peripheral which can be used either as a watchdog
to reset the device when a problem occurs, or as a freerunning timer for application timeout management.
1
The independent watchdog is used to detect and resolve
malfunctions due to software failure. It triggers a reset
sequence when the counter reaches the timeout value.
Since its clock is an independent 32-kHz low-speed
internal RC oscillator (LSI), it remains active even if the
main clock fails. Once enabled, it can only be disabled
by a reset.
One of the main benefits for applications is its ability to
run independently from the main clock. Also, the IWDG
can be configured through hardware or software using
option bytes.
And the IWDG counter can be frozen during Standby or
Stop mode.
2
The IWDG features a 12-bit free-running downcounter. It
is clocked by an independent low-speed internal RC
oscillator at 32 kHz.
When the IWDG is activated, a reset is generated if the
downcounter reaches zero or when the downcounter is
reloaded outside the window (if this option is enabled).
3
The IWDG registers are located in the CORE voltage
domain while its functions are in the VDD voltage
domain. This architecture makes it possible for the IWDG
to continue operating in Stop and Standby modes.
The 8-bit prescaler is used to divide the LSI oscillator
frequency.
When the IWDG is started, the 12-bit counter starts
counting down from the reset value of 0xFFF.
To refresh the IWDG counter, the Key value (0xAAAA)
must be written in the Key register to reload the counter
value.
If the downcounter reaches the end of the count value
(0x000), a system reset is generated.
If the window option is enabled, the counter must be
refreshed inside the window; otherwise, a system reset is
generated.
4
The IWDG hardware is enabled by the device’s option
bytes. If enabled, the watchdog automatically starts at
power on.
To prevent any reset, the Key register must be refreshed
at regular intervals before the counter reaches 0 and
within the window, if this option has been selected.
5
The IWDG software start is configured in a few steps.
• The first step is to write the Key register with value
0x0000 CCCC which starts the watchdog.
• Then remove IWDG register protection by writing
0x0000 5555 to unlock the key.
• Set the IWDG prescaler in the IWDG_PR register by
selecting the prescaler divider feeding the counter
clock.
• Write the reload register (IWDG_RLR) to define the
value to be loaded in the watchdog counter.
After accessing the previous registers, it is necessary to
wait for the IWDG_SR bits to be reset in order to confirm
that the registers have been updated.
• Two options are now available: enable or disable the
IWDG window option.
• To enable the window option, write the window
value in the IWDG_WINR register.
6
•
Otherwise, refresh the counter by a writing
0x0000 AAAA in the Key register to disable the
window option.
6
The IWDG time base is prescaled from the LSI clock at
32 kHz. The IWDG_PR prescaler register can divide the
LSI clock frequency by 4 up to 256. The watchdog
counter reload value is a 12-bit value written in the
IWDG_RLR register.
A formula can be used determine the IWDG timeout. The
IWDG time is based on the LSI period and its prescaler,
as well as the selected watchdog counter reload value.
Given the parameter limits, the IWDG timeout value can
be between 125 µs up to 32.8 s.
Once a reset is generated by the IWDG peripheral, the
corresponding flag, IWDGRSTF, is set in the RCC_CSR
register to inform the source of the reset.
7
When the microcontroller enters Debug mode (core
halted), the IWDG counter either continues to work
normally or stops, depending on the DBG_IWDG_STOP
configuration bit in the DBG module (MCU APB1 freeze
register 1).
8
The IWDG can be active in all running and low-power
modes, except in Shutdown mode where it is powereddown. Nevertheless, in Stop1, Stop2 and Standby
modes, the watchdog counter can be frozen by
programming the corresponding bits in the
FLASH_OPTR register.